This invention relates to x-ray collimators for use in computed tomography systems and the like and specifically to a collimation system for correcting errors in the x-ray fan beam location and angle of incidence with the detector mechanism resulting from misalignment of the position of the x-ray tube focal spot.
Computed tomography systems, as are known in the art, typically include an x-ray source collimated to form a fan beam directed through an object to be imaged and received by an x-ray detector array. The x-ray source and detector array are orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray source and detector array may be rotated together on a gantry within the imaging plane, around the image object, and hence around the z-axis of the Cartesian coordinate system. Rotation of the gantry changes the angle at which the fan beam intersects the imaged object, termed the "gantry" angle.
The detector array is comprised of detector elements each of which measures the intensity of transmitted radiation along a ray path projected from the x-ray source to that particular detector element. At each gantry angle a projection is acquired comprised of intensity signals from each of the detector elements. The gantry is then rotated to a new gantry angle and the process is repeated to collect an number of projections along a number of gantry angles to form a tomographic projection set.
Each acquired tomographic projection set may be stored in numerical form for later computer processing to reconstruct a cross sectional image according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The x-ray source is ordinarily an x-ray "tube" comprised of an evacuated glass x-ray envelope containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by means of a high voltage across the anode and cathode. The voltage applied across the anode and cathode, the current flowing between the anode and cathode, and the duration of the exposure, for a given x-ray procedure, is termed the "exposure technique".
The efficiency of energy conversion in generating x-rays is low, and as a consequence, considerable heat is developed in the anode of the x-ray tube. For this reason, the anode may be rotated at high speeds so that the focal spot constantly strikes a new and cooler area of the anode. Even so, the surface temperature of the anode may rise as high as 2000.degree. C. during the acquisition of the projections for a series of tomographic projection sets and the anode supporting structure including the shaft on which it rotates may rise to 400.degree. C. or more.
As the x-ray source heats up, thermal expansion of the anode supporting structure results in movement of the focal spot relative to the glass envelope of the x-ray tube and movement of the fan beam. The focal spot may move as much as 0.25 mm (0.01 inch) due to thermal expansion during the acquisition of a series of tomographic projections.
The anode shaft is aligned with the z-axis, about which the gantry rotates, to prevent gyroscopic torques from acting on the rotating anode during movement of the gantry. Thermal expansion of the anode support structure therefore tends to move the focal spot along the z-axis. With a fixed collimator position, movement of the focal spot in the z-axis sweeps the fan beam in the opposite direction along the surface of the detector array.
Another source of motion of the focal spot is mechanical stress of the gantry and rotating anode as the gantry rotates. This stress results from the changing angle of gravitational acceleration and the changing magnitude of centripetal acceleration as a function of the rotational velocity of the gantry, acting on the gantry and anode. These resulting forces contribute up to 0.25 mm (0.01 inch) of additional focal spot motion.
The detector array may be an ionization type detector or solid state detector as are known in the art. Both detector types exhibit changes in their sensitivity to x-rays as a function of the position of the fan beam along their surface. Accordingly, movement of the fan beam as a result of thermal drift or mechanical deflection of the x-ray source focal spot may change the strength of the signal from the detector array. Such changes in signal strength during the acquisition of a tomographic projection set produce ring like image artifacts in the resultant reconstructed image.
With a fixed collimator position, movement of the focal spot in the z-axis also affects the alignment of the fan beam with the imaging plane. The mathematics of image reconstruction assumes that each acquired projection is taken within a single plane. Lack of parallelism of the fan beam with the imaging plane will also produces shading and streak image artifacts in the reconstructed image. Also, for small slice widths, the misalignment due to motion induced stress on the gantry and anode may significantly enlarge the effective slice width of images reconstructed from opposing but misaligned views. This motion induced misalignment will reduce contrast resolution for small imaged objects, such as lesions, making them harder to detect. In addition, the spatial resolution of the CT imaging system will be reduced for high frequency features at oblique angles to the slice.